The hydrogenation of all or a part of the nitrile groups to amino groups allows the further chemical modification of the polymers and thus to adjust their properties in a very wide range.
Two typical techniques are known for hydrogenation of polymers that are based on homogeneous and heterogeneous catalysts. Although homogeneous hydrogenation provides high conversions, it sometimes causes chain scission and often leads to polymer contamination with the metal due to catalyst extraction difficulties. On the other hand, heterogeneous hydrogenation often yields moderate conversions but is sometimes preferable to homogeneous hydrogenation due to reduced chain scission, no contamination of the polymer, and the ease of catalyst extraction, separation and regeneration. Furthermore, supported heterogeneous transition and noble metal catalysts are recyclable and produce less byproducts than homogeneous extraction products.
Although the hydrogenation of nitriles to produce the corresponding amines has been studied extensively in the low molecular weight range, inter alia it is described in Homogeneous and Heterogeneous Hydrogenation of Nitriles into a liquid phase: Chemical, Catalytic and Mechanistic Aspects. C. de Bellefon, P. Fouilloux, Catal. Rev. Sci. Eng. (1994) 36 (3), 459-506 there are no indications on the transferability of said method to the hydrogenation of nitrile-containing polymers in general and on nitrile rubber in particular. The hydrogenation of nitrile groups of high molecular weight compounds is much less investigated. According to the Journal of Applied Polymer Science (2001) 79, 1618-1626 the hydrogenation of nitrile groups of nitrile-butadiene rubber (NBR) leads to an increase in the molar mass and gelling of the polymer
In U.S. Pat. No. 2,456,428 the hydrogenation of polymeric nitriles with at least 4 units was described with any of known hydrogenation catalysts (metallic nickel, colloidal platinum, finely dispersed palladium, copper chromite, oxides or sulfides of Cr, W or Mo; preferably Raney-Nickel). As an example a nitrile-styrene rubber was hydrogenated using a Raney Ni catalyst in benzene with addition of ammonia. But no information was given about the molecular weight before and after the hydrogenation and no quantitative analysis of the conversion of nitrile groups to amino groups was done.
The hydrogenation of the butadiene-acrylonitrile co-polymer with molecular weights up to 100,000 by a two stage process wherein first, the C═C groups are hydrogenated on Pd/C and in the following step the CN fragments are hydrogenated on Ni Raney catalyst is disclosed in U.S. Pat. No. 2,585,583. However, the drastic conditions of 250° C. and 930-950 atm do not allow commercial use of this method. Furthermore, U.S. Pat. No. 2,585,583 described that polymers with a higher molecular weight than 100000 cannot be hydrogenated in a clear fashion.
In U.S. Pat. No. 6,121,188, the hydrogenation of the nitrile groups of nitrile rubber was effected in the presence Co Raney with a particular mesoporous structure. In this case a 15% solution of nitrile rubber having a molecular weight of 3000 was hydrogenated in THF. It is further stated that by said method, the reduction of nitrile groups occurs first and the C═C double bond is hydrogenated thereafter.
U.S. Pat. Nos. 8,119,556 and 7,767,859 disclose methods for producing a low-molecular primary amine by hydrogenation of the corresponding nitrile in the presence of a hydrogenation catalyst. The hydrogenation catalyst contains at least one metal selected from the group consisting of nickel, cobalt and iron and has to be pretreated with at least one agent selected from the group consisting of hydrocarbons, alcohols, ethers, esters and carbon monoxide at 150 to 500° C. before its use in the hydrogenation of nitrile. However, iron-containing catalysts are not preferred and no examples thereof are actually shown.
The use of Raney nickel doped with iron chromium, and zinc as a catalyst for the hydrogenation of low-molecular dinitrile compounds is disclosed in US 2011230681.
JP2002201163 and JP2002205975 describe the continuous hydrogenation of low-molecular nitriles to their corresponding primary amines in a liquid phase over a suspended, activated Raney catalyst based on an alloy of aluminum and at least one transition metal selected from the group consisting of iron, cobalt and nickel, and, if desired, one or more further transition metals selected from the group consisting of titanium, zirconium, chromium and manganese. The hydrogenation is carried out in the absence of ammonia and basic alkali metal compounds or alkaline earth metal compounds.
Hitherto, data on hydrogenation of polymers containing amino groups by heterogeneous catalysis in general is scarce and available data either is limited to the hydrogenation of low-molecular polymers or to the use of mainly noble metals, Co Raney or Ni Raney catalysts. In particular, no actual example of a successful hydrogenation of nitrile side groups of polymers with high molecular weights Mw (weights higher than 200,000) on a heterogeneous catalyst in general and a supported Fe(0) catalyst in particular, has been disclosed.